The invention relates to a high-pressure discharge lamp comprising a translucent lamp vessel sealed in a vacuum-tight manner containing an ionizable gas filling a pair of electrodes within the lamp vessel face each other and are connected to a respective current-supply conductor extending through the wall of the lamp vessel. The electrodes of the pair each have a wound wire of mainly tungsten and the distance between the wound wires being substantially equal to the distance between the electrodes.
Such a high-pressure discharge lamp is known from British patent specification No. 1,591,617.
The known lamp is intended to be operated in a position in which the discharge path, i.e. an imaginary straight line through the electrodes, is at least substantially horizontal. In the known lamp, the wound wire of each of the electrodes encloses a quantity of material favouring electron emission (emitter material). This material is sputtered during operation of the lamp. In order to prevent the material from being mixed with the ionizable gas filling or being deposited on the wall of the lamp vessel, the lamp has cavities which are recessed in the seals of the lamp vessel and in which the electrodes are arranged. The electrodes are deformed so that their end face is in line with the inner surface of the wall of the lamp vessel.
In lamps whose electrodes enclose emitter material, the discharge terminates on an electrode at the stage when this electrode is acting as the cathode at a point which is rich in emitter material. When the emitter material at this point becomes depleted, the discharge arc jumps to another point. The point at which the discharge arc terminates on an electrode at the cathode stage is therefore strongly influenced by the presence of emitter material. In emitterless electrodes, this influence is consequently absent.
When the point of termination of the discharge arc on an electrode jumps to another position, this may result in a variation of the value of the luminous flux emitted by the lamp.
If a high-pressure discharge lamp is operated in a position in which the discharge path is horizontal, the discharge arc shifts so as to be curved upwards under the influence of a flow of gas in the lamp vessel due to temperature differences. With comparatively wide lamp vessels, the discharge arc is curved more strongly than with comparatively narrow lamp vessels. The points of termination of the discharge arc on the electrodes are also shifted towards the upper side of the electrodes in a horizontal operating position. In lamps with emitterless electrodes, in contrast with lamps having electrodes with emitters, a stationary termination of the discharge arc on the electrodes may therefore be expected in a horizontal operating position.
However, it has been found that the lamps having emitterless electrodes, which are operated with the discharge path in a horizontal position at an alternating voltage at the current supply frequency, can exhibit substantial variations of the value of the luminous flux, which occur at the frequency of the alternating voltage. These variations become manifest as flickering of the lamp, which can be very annoying.